Electronic locks are characterised in that they have a mechanical lock that physically secures the door to the frame and having certain electronic means for authorising the use of that lock. The electronic means include a reader that permits to read data from a coded medium that can take different forms and different technologies, such as for example a magnetic card, a proximity card or a key with memory. We will in general name these coded media as keys. When a key with valid data is presented, the electronic control permits the outer handle to operate one of the shafts of the mechanical lock, either by means of releasing a latch that was preventing the handle from turning or by means of activation of a clutch that connects the shaft of the handle to the shaft of the lock.
Mechanical locks can have one or several shafts. Some open the catch bolt and are usually operated by a handle or knob. Others close or open a lever and are usually operated by means of cylinder, either with a key, or with a rotating knob. Electronic control can govern the action of one or several of the shafts depending on the applications of the lock. In the description that follows, we do not distinguish between one and another shafts, nor between handles and knobs, using instead the generic term handle.
Solutions based on a latch, though they are simpler, are less secure than those based on a clutch since the latch has to resist all kinds of stresses that are applied to the handle when an attempt is made to force it, while the clutch simply does not transmit those stresses to the lock.
Moreover, the handle in the inner side of the door must always open the lock in order to permit exit in cases of emergency, without the intervention of any electronic control. This feature is called anti-panic.
There are numerous patents on clutch mechanisms for electronic locks housed in the outer escutcheon and which connect the shaft of the outer handle to the shaft that operates the lock, which is permanently connected to the inner handle.
One improvement is to locate the clutch in the inside of the door rather than in the outside. This is a more secure solution since the clutch is then protected from possible manipulations. It is more reliable from the environmental point of view since the inner side usually suffers smaller variations in temperature and humidity. It can also be more aesthetic since the outer escutcheon does not have to house the clutch mechanism.
Patent EP 0 819 810 claims a clutch placed in the inner escutcheon and which actuates the lock via two concentric shafts. The first shaft is connected to the outer handle. The second shaft is connected to the inner handle and to the tumbler of a modified cylinder via which it opens the lock.
When installing the lock, the first shaft is inserted inside the second shaft freely rotating inside it.
For engaging the clutch of both shafts, a piece moves axially, projecting a clutch plate via a slot in the second shaft within a groove in the first shaft. In this way, the two shafts become coupled and permit the outer handle to open the lock.
The problems that appear are due to the necessarily small diameter of the shafts so that they can be compatible with standard locks.
The tumblers of mechanical locks have standardised cross-sections and are usually square with sides of between 7 and 9 mm. The diameters of the concentric shafts that are described can at most be equal to those dimensions in the case of the second shaft and less than those in the case of the first shaft.
The first problem is a consequence of the stresses borne by the mechanism. When the clutch is operated the stress suffered by the plate, the slot of one shaft and the groove of the other is equal to the drive torque of the lock divided by the radius of the shafts. As this radius is very small, the stress is very great and wear takes place very quickly.
The second problem is also caused by the small diameter of the first shaft, which involves that it cannot have more than two slots in order to keep its strength within acceptable minima, since each slot reduces its cross-section. When the clutch is engaged, coupling does not occur between the two shafts until the slot of the second shaft is faced to the one of the grooves of the first shaft. It may therefore be necessary to turn the outer handle under no load up to 180°, which produces an uncomfortable sensation for the user.